This invention relates to electric power supplies, and particularly to a switching power supply featuring provisions for attainment of high power factor with a minimum of noise production.
A conversion from an alternating to a direct current is possible by a rectifying and smoothing circuit comprising a rectifying circuit including a diode and connected to an a.c. power supply, and a smoothing circuit including a capacitor and connected to the rectifying circuit. The rectifying and smoothing circuit possesses the disadvantage, however, of being low in power factor as a result of the fact that the smoothing capacitor is charged only at and adjacent the peaks of the a.c. voltage of sinusoidal waveform. Another drawback is that it is incapable of adjustably varying the d.c. output voltage.
Japanese Unexamined Patent Publication No. 8-154379 represents an improvement of the rectifying and smoothing circuit above. It teaches a switching power supply comprising a rectifying circuit, a smoothing capacitor, a d.c.-to-d.c. converter circuit, and an inductive reactor for a higher power factor. The reactor is electrically connected between the pair of output terminals of the rectifying circuit upon closure of a switch included in the d.c.-to-d.c. converter circuit. The desired improvement in power factor is attained as the current flowing through the reactor varies in amplitude in step with the a.c. input voltage.
This prior art switching power supply has proved to have its own shortcomings. Each time the switch of the d.c.-to-d.c. converter circuit opens, the inductor releases the energy that has been stored thereon, with the result that the current flows through the rectifying circuit for charging the smoothing capacitor. The rectifying circuit includes a diode as aforesaid, to which diode the current due to the energy release from the reactor flows at a repetition rate of as high as 20 to 150 kilohertz. Abrupt changes in the magnitude of the current flowing through the diode are known to give rise to noise with a frequency much higher than that at which the switch is turned on and off. A noise filter, sometimes referred to as line filter, has conventionally been connected between the a.c. input terminals and the rectifying circuit in order to prevent the leakage of the high frequency noise produced by the rectifying circuit and by the d.c.-to-d.c. converter circuit.
The trouble has been the high-frequency noise due to the diode of the rectifying circuit. The total resulting noise has been of annoyingly high level, requiring the provision of several noise filters which have added substantively to the size and manufacturing cost of this type of switching power supply.
The present invention seeks to minimize the noise production of switching power supplies of the kind defined, without in any way adversely affecting their power factor in so doing.
Briefly, the invention may be summarized as a switching power supply capable of translating a.c. voltage into d.c. voltage, comprising a pair of a.c. input terminals for inputting a.c. voltage, a pair of d.c. output terminals for outputting d.c. voltage, a rectifier circuit connected to the pair of input terminals, a transformer having a winding, a rectifying and smoothing circuit connected between the transformer and the pair of d.c. output terminals, an inductor for improvement of the power factor of the input terminals, a reverse blocking diode, a smoothing capacitor connected between the pair of outputs of the rectifier circuit via at least part of the transformer winding, the reverse blocking diode and the inductor, a switch connected between the pair of outputs of the rectifier circuit via at least the inductor and the reverse blocking diode and in parallel with the smoothing capacitor via at least part of the transformer winding, a switch control circuit connected to the switch for on-off control of the switch at a repetition frequency higher than the frequency of the a.c. input voltage, and a bypass capacitor. The bypass capacitor is connected between the pair of outputs of the rectifier circuit and in parallel with the serial circuit of the inductor and the reverse blocking diode and at least part of the transformer winding and the smoothing capacitor. The bypass capacitor is less in capacitance than the smoothing capacitor. The reverse blocking diode has a reverse recovery time shorter than the nonconducting periods of the switch,
The invention particularly features the bypass capacitor, with a capacitance less than that of the smoothing capacitor, which is connected between the pair of outputs of the rectifier circuit. Consequently, unlike the prior art, the current due to energy release from the inductor during the nonconducting periods of the switch does not flow through the rectifier circuit, but through the bypass capacitor taught by the invention. High frequency noise production by the rectifier circuit is avoided as the intermittent current flow through the diode of the rectifier circuit is prevented as above.
The bypass capacitor is so connected, as summarized above, that at least part of the transformer winding is interposed between the inductor and the smoothing capacitor. The smoothing capacitor can then be charged with a relatively low voltage and so need not be of expensive construction for withstanding high voltages.
For further noise reduction an additional capacitor may be connected in parallel with the switch at least via the reverse blocking diode, and with the bypass capacitor via the inductor. This additional capacitor will redound for reduction of high frequency noise due to the operation of the switch.
The above and other objects, features and advantages of this invention will become more apparent, and the invention itself will best be understood, from a study of the following description and appended claims, with reference had to the attached drawings showing the preferred embodiments of the invention.